The present invention relates, in its various aspects, to time-base correction (TBC) of video signals recorded on frequency-modulated carriers in various video signal recording/reproducing systems, and more particularly to apparatuses for recording and reproducing a reference signal used for correcting timing irregularities of video signals reproduced from such frequency-modulated carriers.
Generally, a video signal includes video information arranged in accordance with a synchronizing signal so as to display accurate images on display devices. This synchronizing signal is affected by noise during recording and reproducing through a recording medium, just as any other information signals are, thereby producing time-base errors in the video information. Time-base errors occur mainly due to mechanical elements included in the recording/reproducing apparatus. Temporal lengthening or shortening of the signal owing to time-base errors causes trembling or "jitter" of reproduced television images which is tiring to a person viewing those images.
Especially when an analog video signal is converted to a digital video signal for digital processing, time-base error can cause a variation in the number of samples per horizontal scan line to occur, even though the number of samples for each horizontal scan line is required to be constant. As a result, video information designated for each pixel location on the television screen may move to the left or to the right by one or more pixels per horizontal scan line, so that the spatial phasing of the signal is not coherent. While the spatial incoherency may be tolerable between adjacent horizontal scan lines, it becomes more severe over several horizontal scan lines, so signal processing between frames becomes impossible due to the changing pixel positions.
A time-base-error corrector (hereinafter referred to as a "TBC") is an apparatus that corrects the timing irregularities ("jitter") of video information due to time-base error of the video signal. The time-base-error corrector is employed to remove the time-base errors created during recording and reproducing and to resynchronize the signal more precisely in video recording and reproducing systems.
A conventional TBC used in video recording and reproducing systems will now be described, referring to FIG. 1 of the drawing. In FIG. 1, analog-to-digital converter (A/D converter) 10 samples a video signal entering through input line 5 over respective periods prescribed by a writing clock generator 40, thereby generating a digital video signal. A memory 20 temporarily stores the digital video signal converted in the A/D converter 10 on a first-in/first-out (FIFO) basis. The memory 20 can be constructed using a random-access memory (RAM) with storage capability for several horizontal scan lines, arranged to receive separate write addressing and read addressing. Digital-to-analog converter (D/A converter) 30 converts the digital video signal read-out from the memory 20 to an analog video signal supplied through output line 15. To keep the number of data samples supplied by A/D converter 10 for storage in memory 20 the same for every horizontal scan line, a writing clock generator 40 generates writing clock signals having periods the durations of which are adjustable according to variations of horizontal synchronizing signal periods of the video signal supplied through input line 5. These writing clock signals time the sampling of the video signal on input line 5 by the A/D converter 10. These writing clock signals are also counted for generating write addresses for the memory 20, supposing it to be a RAM arranged for FIFO operation. FIG. 1 does not explicitly show this counter for generating write addresses, which is included in the writing clock generator 40. Reading clock generator 50 generates a reading clock signal having a periodicity that is invariant or substantially invariant. This reading clock signal clocks the reading from the memory 20 during its FIFO operation and times the conversion of the read-out samples to analog video signal by the D/A converter 30. In the writing clock generator 40, a horizontal synchronizing signal separator 41 separates horizontal synchronizing pulses from a video signal supplied through input line 5 for application to a phase comparator 42, there to be compared to the phase of a frequency-divided write clock signal supplied from a frequency divider 44 for generating a control voltage corresponding to the phase difference. Voltage-controlled oscillator 43 (hereinafter referred to as "VCO") generates write clock signals having adjustable frequency and phase depending on the control voltage applied thereto from the phase comparator 42. These write clock signals are supplied to the A/D converter 10, to the FIFO memory 20 and to the frequency divider 44. The frequency divider 44 divides the frequency of the write clock signals in order that the clocks have the same frequency as that of the horizontal synchronizing signal and supplies the clocks to phase comparator 42. This closes an automatic frequency and phase control (AFPC) feedback loop that regulates the write clock signals from the VCO 43 to be a prescribed multiple of the horizontal scan line rate of the video signal on input line 5 by the A/D converter 10, tracking the time-base error in that video signal. The frequency divider 44 commonly includes a counter, which also supplies a count that provides the write clock signals for the memory 20 or that has an offset added to it to provide the write clock signals for the memory 20. This makes the write clock signals track the time base variations of the horizontal synchronizing pulses that the synchronizing signal separator 41 separates from the video signal supplied through input line 5.
Accordingly, the conventional TBC shown in FIG. 1 samples a video signal a prescribed number of times during each horizontal scan line and writes the samples of digitized video signal into the memory 20. Thereafter, such video data stored in the memory 20 is read therefrom in accordance with the reading clock signals having a periodicity that is invariant (or substantially so) and is converted to an analog video signal in D/A converter 30 to correct the time-base error in the video information of the video signals supplied on the output line 15. The reading clock signals are normally read addresses generated by a read address counter cyclically counting the clock signals supplied from a stable oscillator, such as a crystal-controlled type. Time-base-error correction is provided by the memory 20 delaying the digitized video signal in amount controlled by the difference in write and read addresses applied thereto.
In the FIG. 1 TBC, the AFPC feedback loop that regulates the write clock signals from the VCO 43 to be a prescribed multiple of the horizontal scan line rate of the video signal on input line 5 has a time constant which is at least greater than the horizontal synchronizing signal period and is usually of two or three times the horizontal synchronizing signal period. This time constant is established by low-pass filtering in the output circuit of the phase comparator 42. When the frequency transfer characteristic of a recording system is very narrow as compared with the band of an original signal, the rise time and the fall time of the horizontal synchronizing signal are stretched, which makes it difficult to time precisely the ending of each horizontal scan line and the beginning of the next horizontal scan line. Furthermore, because of tracking error in the AFPC loop, which varies somewhat over time because of noise and other factors, the frequency of the write clock signals cannot be changed precisely according to the variation of the horizontal synchronizing signal period. For these reasons, time-base-correction of a video signal can not be carried out with the desired degree of precision by the conventional TBC shown in FIG. 1.
It is observed that the higher the frequency of a single-frequency TBC signal, the greater are the number of radians of phase shift that that signal will exhibit as a result of time-base error. Color bursts are broadcast during horizontal blanking intervals to be used for synchronization of the local color oscillators of television receivers. These horizontal blanking intervals correspond to the times that horizontal retrace occurs in the kinescopes of television receivers, when the video signals applied to the kinescopes are "blanked" by being made blacker than normal black level. Color bursts are not broadcast during certain horizontal blanking intervals that occur in vertical blanking intervals, however. These vertical blanking intervals correspond to the times that vertical retrace occurs in the kinescopes of television receivers, when the video signals applied to the kinescopes are blanked. The color bursts are at color subcarrier frequency (3.58 MHz for the NTSC broadcast standard used in the United States of America). AFPC of a local color oscillator to these bursts of color subcarrier at an odd half-multiple of horizontal line scan rate is substantially more accurate than attempting to AFPC to horizontal synchronizing pulses.
The broadcasting of color burst for synchronization of the local color oscillators of television receivers is perhaps suggestive of video recording using an FM carrier in which a TBC signal is used that comprises intermittent bursts of a single frequency timed to occur during horizontal blanking intervals. Indeed, where the video recording system is sufficiently wideband in nature to permit the recording of a composite video signal, the use of color bursts themselves as TBC signals might be contemplated. The standard duration of color bursts is sufficiently long to allow satisfactory operation of AFPC systems where there is not much time-base error (e.g., in TV receivers receptive of TV broadcast or cablecast transmissions). However, I have found operation of AFPC systems locking to color burst to be unsatisfactory or only marginally satisfactory in video tape recording where there is substantial time-base error. The duration of each color burst is shorter than necessary for satisfactory operation of the AFPC in the time-base corrector. Also, the largest need for AFPC correction occurs during the vertical retrace interval, just after head switching occurs in the helical recording system commonly used in video recording. Color burst is not transmitted during a substantial portion of the vertical retrace interval.
An aspect of my invention is the recording of TBC signals comprising intermittent bursts of a single frequency timed to occur during horizontal blanking intervals, but of longer duration than standard color bursts. Another aspect of my invention is the recording of TBC signals comprising sustained transmission of a single frequency, respective ones of which transmissions are timed to occur during corresponding ones of consecutive vertical blanking intervals. These longer duration TBC signals permit more satisfactory operation of the AFPC in the time-base corrector. Where the video recording system has sufficient bandwidth to permit the recording of composite video signal, the single frequency used in the TBC signal can be color subcarrier frequency, but better AFPC operation can be obtained using a TBC signal of still higher frequency.
Many, if not most, video recording systems using an FM carrier have insufficient bandwidth to permit the recording of composite video signal. Commonly, the composite video signal is separated into luminance-signal and chrominance-signal components, the luminance signal is used to modulate the frequency of a luma carrier to generate an upper band of the signal recorded on the magnetic recording medium, and the chrominance signal is down-converted in frequency to generate a color-under signal that is used as a lower band of the signal recorded on the magnetic recording medium. In such video recording systems the bandwidth of the luminance signal is generally restricted to only 2.5 to 3 MHz. I find that operation of the AFPC for the oscillator used to generate write signals for the TBC memory is only marginally satisfactory for a TBC signal comprising intermittent bursts of a frequency lower than color subcarrier, which bursts are timed to occur during horizontal blanking intervals.
I find that a higher-frequency TBC signal (of, say, 5 MHz) can be accommodated, even though the signal modulating the frequency of the luma carrier is restricted to only 2.5 to 3 MHz. Rather than including the TBC signal in the signal modulating the frequency of the luma carrier, during retrace or blanking intervals it is additively mixed (i.e., frequency-division-multiplexed) with the FM luma carrier and the AM color-under signal. After reproducing the frequency-division-multiplex signal recorded on the recording medium, the AM color-under signal is selected by a lowpass filter that does not respond to the FM luma carrier and the TBC signal, and the FM luma carrier and the TBC signal are selected by a highpass filter that does not respond to the AM color-under signal. The luminance signal is recovered by an FM detector following the highpass filter. Supposing that the TBC signal is transmitted only during retrace intervals, it can be blanked out of the luminance signal recovered by the FM detector. However, if the TBC signal is of relatively small amplitude compared to the FM luma carrier with which it is additively mixed, the capture phenomenon associated with the FM detection process will suppress the TBC signal in the FM detector response. The TBC signal is suppressed in the FM detector response even during blanking intervals, so it does not interfere with synchronizing signal separation from the FM detector response. The TBC signal is separated from the highpass filter response by appropriate gating, by frequency-selective filtering, or by a combination of appropriate gating and frequency-selective filtering.
While the TBC signal could be at a frequency above the frequency band which the sidebands of the frequency-modulated luma carrier occupy, this would require more expensive heads with narrower gaps and better recording tape. So, a better commercial solution is to place the TBC signal within the same frequency band as the sidebands of the frequency-modulated luma carrier. Making the TBC signal the same frequency as color subcarrier (3.58 MHz for the NTSC broadcast standard used in the United States of America) in VHS-type video recording complicates the frequency-selective filtering for separating the TBC signal from the 3.4-3.68 MHz principal sideband frequencies of the luma carrier generated responsive to sync pulses. Accordingly, I prefer to use a TBC signal that is at a frequency above the principal sideband frequency of the luma carrier when modulated past the white end of the gray scale, but is still within the extended upper sideband response that is recorded on the video recording tape. I find a TBC signal of around 5 MHz to be desirable. If one desires, with such a TBC signal above normal video frequencies, trap filtering can be used to remove the TBC signal from the luminance signal reproduced from the video tape, rather than removing the TBC signal just by selectively blanking the luminance signal.